1. Field of the Invention
The present invention relates to aluminum-lithium based alloy products, particularly those suitable for use as structural members in aircraft construction, such as in bulkhead, spars, wing skin, frames, extruded structural members, and fuselage applications, as well as other applications where a combination of high strength and high fracture toughness are typically desirable and/or required.
2. Description of Related Art
In many industries and particularly in the aircraft industry, reducing the weight of structures has always been a concern. One effective way of doing this is to reduce the density of aluminum alloys used in such structures. It is well known in the art that aluminum alloy densities may be reduced by the addition of lithium. However, it is also known that some problems arise when lithium is added to aluminum based alloys. One of the problems encountered is the possible decrease in ductility and fracture toughness.
Most structural applications in the aircraft industry, and particularly applications such as products intended for use in lower wing skin structures, require a high level of strength, as well as a high level of fracture toughness. It is also desirable for aircraft and other similar applications, that ductility and corrosion behavior remain at an acceptable level.
Among aluminum-lithium based alloys, Al—Cu—Li—Mg—Ag alloys are well-known in the prior art for their interesting properties. Specifically, U.S. Pat. No. 5,032,359 discloses an alloy with a broad composition of 2.0 to 9.8 wt. % of an alloying element, which may be copper, magnesium, or mixtures thereof, the magnesium being at least 0.05 wt. %, from about 0.01 to about 2.0 wt. % silver, from about 0.2 to about 4.1 wt. % lithium, and from about 0.05 to about 1.0 wt. % of a grain refining additive selected from zirconium, chromium, manganese, titanium, boron, hafnium, vanadium, titanium diboride, and mixtures thereof.
U.S. Pat. No. 5,389,165 discloses a preferred composition of 1.10 wt. % Li, 3.61 wt. % Cu, 0.33 wt. % Mg, 0.40 wt. % Ag and 0.14 wt. % Zr. An alloy composition corresponding to such a range was registered at The Aluminum Association in June 2000 as AA 2098. This alloy exhibits high fracture toughness and strength at elevated temperatures, after having been subjected to a specific process. An alloy as disclosed in the '165 patent may be suitable for some thin or medium gauge plate products used in aircraft structures, but may be less suitable for use as thick gauge plates, because of rather low mechanical properties in the ST direction.
Another aluminum-lithium based alloy has also been proposed for thick gauges. This alloy, registered at The Aluminum Association as AA 2297 in August 1997, contains lithium, copper, manganese, and optionally magnesium, but no silver. U.S. Pat. No. 5,234,662 discloses a preferred composition of 1.6 wt. % Li, 3.0 wt. % Cu, 0.3 wt. % Mn, 0.12 wt. % Zr. The alloy, produced in thick gauges, exhibits a good combination of low density, strength, toughness, fatigue resistance and corrosion resistance.